The present invention relates to a system for supplying air at a controlled flow rate, pressure and temperature to enable control of the clearances between the blade tips and a surrounding casing in rotating machinery and more particularly relates to a system for controlling the flow of high pressure air at a control flow rate, pressure and temperature to control clearances between turbine blade tips and a surrounding turbine casing of a heavy-duty gas turbine during various operating modes of the turbine.
While the present invention is described in this application in connection with a gas turbine, it will be appreciated that the invention is also applicable more generally to machinery having parts rotating past stationary surfaces, e.g., compressor blades rotating relative to a surrounding casing. The efficiency of a gas turbine can be increased by reducing the clearance between the tips of the turbine blades and their surrounding casing. However, to accommodate differential rates of thermal growth during start-up, acceleration, warm-up and steady-state operating modes, the turbine must be constructed with relatively large clearances between the rotating blade tips and the surrounding casing. For increased efficiency, these different modes of operation require thermal control of the casing, for example, during start-up, to cause the casing to expand, and during steady-state operations to cause the casing to contract to minimize the clearance with the blade tips. Previously, it has not been customary to provide turbomachinery blade tip clearance control in heavy-duty gas turbines. Simple clearance control systems have been employed in aircraft engines. These systems employ air bled from the engine""s compressor or fan to cool the casing surrounding the turbine blades or to heat the rotor carrying the turbine blades. The problem confronted by the present invention is to provide a system for providing high pressure air at a regulated flow rate, pressure and temperature to heat or cool the turbine casing in accordance with various operating modes of the turbine.
In accordance with the present invention, there is provided a closed-cycle compressed air system for supplying air to and transferring heat to or from a casing surrounding the rotating blades of the turbine to control the clearance between the casing and the tips of the rotating blades. During gas turbine start-up, the system heats the turbine casing, causing it to expand more rapidly than the rotor and accompanying blades to ensure that the blade tips do not contact the surrounding casing. During steady-state gas turbine operation, the system cools the turbine casing, reducing the clearance between the blade tips and the casing, thereby improving the efficiency of the turbine. Following turbine shutdown, the system can heat the turbine casing to maintain clearances (by ensuring that the casing remains relatively hot while the turbine blades cool down naturally).
In a preferred embodiment of the present system, air is supplied under pressure through an air control valve to the closed-circuit air system, the air supplied being derived from a charging compressor or an existing pressurized air supply. To circulate the air about the system""s closed circuit, a circulation compressor is provided in series with an upstream heat exchanger and a downstream air heater, with the gas turbine casing being located in series downstream of the air heater and upstream of the heat exchanger. The air heater heats the air exiting the circulation compressor for flow to the turbine casing. The heat exchanger is in heat exchange relation with a coolant supply whereby air exiting the casing and supplied to the circulation compressor is cooled.
In operating the system, two nominal temperature settings are employed for discharging air from the system to the turbine casing. During start-up, a high system discharge temperature is required to heat the casing. Consequently, the heater, downstream of the circulation compressor, heats the air in the air stream to maintain the required high system discharge temperature. During steady-state operation, the system cools the air supplied the turbine casing. Hence, a lower system discharge temperature is required. To provide the lower system discharge temperature, the heater is turned off and the system discharge temperature is regulated by controlling the supply of coolant to the heat exchanger whereby the temperature of the air exiting the heat exchanger and supplied the circulation compressor is predetermined. At all times, the system maintains the temperature at the inlet of the circulation compressor below a safe limit for operation of the compressor.
In a preferred embodiment according to the present invention, there is provided, in a system for controlling clearance between the tips of blades of rotating machinery and a surrounding casing during start-up and steady-state operating modes of the machinery wherein air is circulated in the surrounding casing at predetermined pressure, temperature and flow rate, a method of operating the system comprising the steps of heating the air prior to circulating the air through the casing during start-up to expand the turbine casing and cooling the circulating air exiting through the casing during steady-state operation, whereby clearances between the casing and the blade tips are maintained in both start-up and steady-state operating modes.
In a further preferred embodiment according to the present invention, there is provided a system for controlling clearance between the tips of blades of rotating machinery and a surrounding casing comprising a closed air circuit in communication with the casing of the rotating machinery and including a heat exchanger, a circulation compressor for circulating air in one direction through the air circuit, and an air heater connected in series with one another and with the casing, the air heater being disposed downstream of the circulation compressor and upstream of the casing, the heat exchanger lying downstream of the casing and upstream of the compressor, a bypass passage connected in the air circuit on opposite sides of the heat exchanger and a valve in the bypass passage, the air heater, when energized during start-up of the machinery, supplying heated air to the casing at a first temperature to expand the casing with the bypass valve open to flow air in the air circuit through the passage bypassing the heat exchanger, the bypass valve being closed during steady-state operations of the rotating machinery to enable the heat exchanger to supply cooled air to the casing at a second temperature lower than the first temperature to contract the casing and maintain a desired clearance between the casing and the blade tips.
Accordingly, it is a primary object of the present invention to provide a system for supplying high pressure, high temperature air at a controlled flow rate for circulation through a turbine casing to control the clearances between the tips of the blades of the rotating machinery and the casing in a controlled variable manner to change the clearances in accordance with the various modes of operation.